Polarization management is generally a critical requirement for state-of-the-art integrated photonic systems. Conventional photonic structures exhibit a high degree of asymmetry in the vertical direction, either by design or by limitations of the particular fabrication method employed. Also, the achievable refractive index contrast, which affects the achievable compactness of the photonic structures desired, is typically small, which has the particular disadvantage of poor temperature stability, and cannot be brought to very small dimensions as a restriction of methods employed. Key building blocks such as polarizers and polarization beam-splitters (PBS) to date have achieved operation over only limited optical bandwidths, thus limiting their uses.
Polarization management in modern integrated photonics is conducted in a variety of ways, depending on the processes available or the platforms considered. Most commonly, a polarization-filtering effect is achieved using a metal cladding or grating on the surface of a waveguide, which introduces large losses for one polarization but not for the other polarization. Alternatively, shallow etching is applied to one area of a waveguide such that the transverse-magnetic (TM) light will leak out. These methods generally require significant additional processing on a wafer in order to achieve polarization. Furthermore, if both TM and transverse-electric (TE) polarization are desired, the amount of additional processing increases further since separate designs are needed to process TE polarized light and the TM polarized light. Additionally, large losses for the “pass” polarization can result from their interaction with metal claddings, or from transitions between shallow and deep etched regions. The demonstrated bandwidths of conventional integrated polarizes and PBS are fairly limited, generally not exceeding 100 nm in the telecom band.
Concerning integrated PBS devices, state-of-the-art approaches are typically precision-engineered directional couplers that selectively couple one polarization into a specific output channel but not the other polarization. Although they can be quite compact, they generally either require difficult-to-fabricate geometries (e.g., two waveguides of different height next to each other), or complicated additional processing steps. Still, the bandwidths are typically limited to <300 nm even in simulated designs.
In the telecommunications market, polarization diversity functions are often implemented in fiberized components that are bulky and expensive. In remote optical sensing, it is often desirable to extract information on the polarization dependence of a received signal from a target. Additionally, spectroscopic analysis may be needed simultaneously with the information extraction. Such processing can generally be achieved with free-space optics, but such free-space optics systems are alignment sensitive and expensive to implement.